Peptides are used in the fields of pharmaceuticals, foods and various other fields. For example, since L-alanyl-L-glutamine has higher stability and water-solubility than L-glutamine, it is widely used as a component of fluid infusion and serum-free media.
Chemical synthesis methods, which have been known as methods for producing peptides, are not always easy. Known examples of such methods include a method that uses N-benzyloxycarbonylalanine (hereinafter, “Z-alanine”) and protected L-glutamine (see Bull. Chem. Soc. Jpn., 34, 739 (1961), Bull. Chem. Soc. Jpn., 35, 1966 (1962)), a method that uses Z-alanine and protected L-glutamic acid-γ-methyl ester (see Bull. Chem. Soc. Jpn., 37, 200 (1964)), a method that uses Z-alanine ester and unprotected glutamic acid (see Japanese Patent Application Laid-open Publication No. H1-96194), a method that involves synthesis of an N-(2-substituted)-propionyl glutamine derivative as an intermediate from a 2-substituted-propionyl halide as a raw material (see Patent Application Laid-open Publication No. H6-234715).
However, since all of these methods require the introduction and elimination of protecting groups or the use of an optically active intermediate, they are not considered to be adequately satisfactory in terms of their industrial advantages.
On the other hand, widely known examples of typical peptide production methods using enzymes consist of a condensation reaction that uses an N-protected, C-unprotected carboxy component and an N-unprotected, C-protected amine component (hereinafter, “Reaction 1”), and a substitution reaction that uses an N-protected, C-protected carboxy component and an N-unprotected, C-protected amine component (hereinafter, “Reaction 2”). An example of Reaction 1 is a method for producing Z-aspartylphenylalanine methyl ester from Z-aspartic acid and phenylalanine methyl ester (see Japanese Patent Application Laid-open Publication No. S53-92729), while an example of Reaction 2 is a method for producing acetylphenylalanylleucine amide from acetylphenylalanine ethyl ester and leucine amide (see Biochemical J., 163, 531 (1977)). There have been reported very few research examples of methods that use an N-unprotected, C-protected carboxy component. An example of a substitution reaction that uses an N-unprotected, C-protected carboxy component and an N-unprotected, C-protected amine component (hereinafter, “Reaction 3”) is described in International Patent Publication WO 90/01555. For example, a method for producing arginylleucine amide from arginine ethyl ester and leucine amide may be mentioned of. Examples of substitution reactions that use an N-unprotected, C-protected carboxy component and an N-unprotected, C-unprotected amine component (hereinafter, “Reaction 4”) are described in European Patent Publications EP 278787A1 and EP 359399B1. For example, a method for producing tyrosylalanine from tyrosine ethyl ester and alanine may be mentioned of.